1. Field of the Invention
The present invention relates to a radiation sensor and a radiation image detection apparatus.
2. Description of the Related Art
In the field of medicine, use is being made of a radiation sensor which irradiates a human body with a radiation such as X-ray, detects the intensity of the radiation that has transmitted into the human body, and thereby detects the images of the inside of the human body. One type of such a radiation sensor is a system which first enters a radiation that has transmitted into the human body, into a phosphor to thereby convert the radiation into visible light, converting this visible light into electrical signals, and then extracting the electrical signals to the outside.
As such a radiation sensor, for example, there is known a radiation sensor in which a impact-resistant phosphor layer composed of a phosphor that is sensitive to radiation, such as gadolinium oxysulfide; an upper electrode; a lower electrode; and a impact-resistant support which is disposed between the upper and lower electrodes, and has an organic photoelectric conversion layer that absorbs the light emitted by the radiation incident to the phosphor layer to convert the light to charges, and a charge detection layer that includes a storage capacitor and a thin film transistor unit for reading the charges generated in the organic photoelectric conversion layer from each of the pixels for image detection (see Japanese Patent Application Laid-Open (JP-A) No. 2004-172375). Furthermore, since a thin film transistor which has an oxide semiconductor active layer containing zinc oxide, may be formed into a film at low temperature, an insulating substrate having impact resistance, such as a plastic plate or a plastic film, may be used (see JP-A No. 2004-172375 and JP-A No. 2006-165530).
However, a thin film transistor which uses an oxide semiconductor, particularly an oxide semiconductor containing indium (In), gadolinium (Ga) and zinc (Zn) among others, as an active layer has a property that the threshold voltage shifts to a negative value when it is irradiated with light having a wavelength of 460 nm or less [see Chiao-Shun Chuang, et al. (University of Michigan), “P-13: Photosensitivity of Amorphous IGZO TFTs for Active-Matrix Flat-panel Displays,” “SID 08 DIGEST,” pp. 1215-1218 (2008)]. Therefore, when the thin film transistor is repeatedly irradiated with light having a wavelength of 460 nm or less, threshold shifts accumulate, and the operation of the thin film transistor becomes unstable, so that the operation as a radiation sensor may become unstable.